1. Field of the Invention
The present invention generally relates to spherical antennas and, more particularly, to wide-scanning, spherical antennas having a fixed, compact main spherical reflector.
2. Description of the Prior Art
Narrow beamwidth antenna systems are used in applications such as point-to-point communication systems which demand high gain antennas having high resolution. The antenna system of choice usually employs a large main reflector antenna because of its high gain and feed system simplicity. In many, if not most, applications the main beam of the antenna radiation pattern must be scannable over a region of space to permit spacial directivity and control of transmitted or received electro-magnetic waves. In communications applications varying traffic demands dictate scan coverage. In remote sensing applications a scanning scenario is employed to collect data over a desired observation region. Narrow beamwidth antennas are often physically large. Scanning by mechanically skewing or moving the entire antenna assembly is difficult and, in many situations, is unacceptable. For example, in space-based systems large-mass mechanical motions would disturb the space platform that might also support other systems which are vibration sensitive. It is, therefore, desirable to have a scanning system which does not involve motion of the main reflector. Such antenna systems typically accomplish scan through mechanical motion of a feed subassembly and/or through electronic means such as a phased array feed.
Performance during scan is, of course, also very important. Traditional directional antenna systems employ a well-focused parabola-shaped main reflector which accomplish scan by either motion of a few feeds, segmental excitation of several displaced feeds, or by phase steering a focal plane feed array. Unfortunately, such scanning systems can experience significant gain loss.
Spherical antenna systems have been developed which use a stationary spherical main reflector and a scanning feed subassembly. Traditional spherical antenna systems have low aperture utilization and poor side lobe and cross polarization characteristics, and are therefore not often used. Aperture utilization relates to the size of the antenna and is the ratio of the physical area of the main reflector to the area that is actually illuminated during scan, and is designate as: ##EQU1## Where .epsilon..sub.u is the aperture utilization factor, D is the physical diameter of the main reflector, and D' is the diameter of the illuminated aperture. The spread of the illuminated aperture and the power distribution of the illuminated aperture remains constant with scan; however, the position of the illuminated aperture on the main reflector surface moves considerably. Thus, over-sizing of the main spherical reflector is necessary to prevent the beam from overshooting or spilling off of the main reflector during extreme scan angles. This results in a poor aperture utilization factor which dictates a physically large antenna system.
There are several types of scanning spherical antenna systems. The simplest is the prime-focus spherical reflector which has a spherical reflector 2 and a feed subassembly 4, as shown in FIG. 1. The "focal point" is not an exact focal point as with a paraboloidal reflector; but rather is a caustic region known as the focal arc, designated by the letter "F". Scan is accomplished by moving the feed along the focal arc. The prime-focus spherical reflector requires a large F/D' ratio to limit the spherical aberration. Hence, scan is achieved at the expense of a large antenna structure with a small aperture utilization factor.
Dual-reflector systems have been developed which use a spherical main reflector and a subreflector which corrects for the spherical aberration and permits a smaller F/D'. This reduces the radius of the spherical main reflector so that the aperture utilization factor is improved somewhat. However, the power distribution on the illuminated portion of the main reflector cannot be controlled; thus, the side lobe and cross polarization performance is poor.
Tri-reflector systems have recently been developed which improve upon the side lobe and cross polarization performance of the dual-reflector systems. Tri-reflector systems include a main spherical reflector and two subreflectors. There have been two methods suggested for determining the shape of the subreflectors: a partial differential equation method and an optimization method. Kildal et at., Synthesis of Multireflector Antennas by Kinematic and Dynamic Ray Tracing, IEEE Trans. Antennas Propagat., October, 1990, Vol.38, No. 10, pp. 1587-1599, discloses a partial differential equation method which uses an approximate numerical solution for a set of partial differential equations derived from kinematic and dynamic ray tracing. The Kildal et al. numerical solution is only an approximate solution because the feed-to-aperture mapping is allowed to float for purposes of calculation. U.S. Pat. Nos. 4,464,666 and 4,516,128 to Watanabe et al. disclose the optimization approach whereby the subreflectors surface shapes are calculated using a synthesis functional expansion. Each basis function in the expansion series satisfies the equal path length condition, correcting for spherical aberration. The coefficients in the series expansion are optimized to achieve the desired aperture distribution. Both the Kildal et al. and Watanabe et al. methods produce subreflectors for tri-reflector antenna systems which decrease the high side lobes and high cross polarization.
All of the foregoing spherical antenna systems have an aperture utilization factor, .epsilon..sub.u, less than unity which leads to low aperture efficiency. Hence, the main reflector needs to be oversized to prevent spillover resulting in relatively large main reflector. Additionally, movement of the suboptic assembly during scan, although simpler than slewing the main reflector, is still relatively mechanically complex.